1. Field of the Invention
The present invention relates to thin film resonators.
2. Description of Related Art
Membrane type piezoelectric resonators are useful for making RF filters and other circuit elements in electronic equipment such as wireless terminals and handsets. Typically, a very thin piezoelectric member, sandwiched between electrodes, is suspended over a cavity. And, while the membranes are very thin, about 2.5 microns for an aluminum nitride membrane in a 2 GHz application, the membranes must span a cavity several hundred microns wide.
Such membranes are prone to bowing, either because of inherent stresses therein or external stress (e.g., an applied voltage, temperature change, handling, etc.). If the membrane bows toward a nearby solid structure, proximity or contact can lessen or destroy any advantages to the structure being initially fabricated as a membrane.
In the thin film resonator according to the present invention, a support structure is provided within the cavity spanned by the piezoelectric membrane. The support structure serves to attenuate or prevent the bowing of the piezoelectric membrane and thus eliminate the problems and disadvantages discussed above with respect to thin film resonators. Alternatively, or additionally, the support structure serves to desirably change the acoustical properties of the piezoelectric membrane. Specifically, according to the embodiments of the present invention, the support structures are formed in the sacrificial layer that is removed to form the cavity. By photolithography and deposition of an etch resistant material, structures such as columns, struts, pegs, etc. are formed in the sacrificial layer. Because the structural elements have a different etch selectivity from that of the sacrificial material, when the sacrificial material is etched away to create the cavity, the support structure elements remain.
Accordingly, these support structures improve the yield and reliability of the manufactured devices by decreasing the occurrence of membrane cracking and breaking. Furthermore, the support structures improve the response of the piezoelectric membrane by preventing unwanted motion that affects the desired electrical signature, and alternatively or additionally, the support structures desirably change the electrical signature.
In another embodiment, the support structures provide for stacking membranes and the formation of a protective cap layer, without the fear of having the membrane negatively affected by possible contact with the cap layer.